In the manufacture of a variety of products, such as polymer matrix composites and vinyl polymers, as well as in the applications of dental or medical resins, the cure or polymerization reactions of polymerizing materials must be adequately monitored and controlled to produce the desired resultant products. Fluorescence techniques are particularly useful for cure monitoring because they are sensitive and adaptable to non-destructive, in-line, real-time monitoring.
One known fluorescence technique for monitoring the cure of an epoxy resin is disclosed in Wang, et al., 27 Polymer 1529 (1986), which is herein incorporated by reference. According to the technique, a trace amount of 1-(4-dimethylaminophenyl)-6-phenyl-1,3,5-hexatriene ("DMA-DPH"), which is a viscosity-sensitive fluorophore, and a trace amount of 9,10-diphenylanthracene ("DPA"), an internal standard fluorophore which is insensitive to viscosity, are dissolved in an epoxy resin. The fluorescence intensities of the viscosity-sensitive fluorophore, DMA-DPH, and the internal standard, DPA, are then measured at various cure times. Finally, the ratio of these intensities, which is insensitive to the change in the shape of the sample or the presence of filler particles, is used to monitor the cure of the epoxy resin. An advantage of this technique is that the use of two fluorophores eliminates inaccuracies in measuring the absolute fluorescence intensity of a viscosity-sensitive fluorophore when the polymerizing material contains filler particles or undergoes polymerization shrinkage. However, the method suffers from the drawback that the two fluorophores may have overlapping fluorescence spectra, thereby necessitating complicated deconvolution of the fluorescence spectra.
U.S. Pat. No. 4,651,011 to Ors and Scarlata, also incorporated herein by reference, discloses another method of determining the extent of cure of a polymerizing material in which the change in fluorescence anisotropy of a fluorophore dissolved in a polymerizing material is measured. In contrast to the above-described method of Wang, et al., the method of Ors and Scarlata utilizes only one fluorophore. However, a major drawback of the method is that it requires the use of a complex optical system. In particular, the method uses polarized exciting radiation, together with polarizers for separating fluorescence intensities in two mutually perpendicular directions.